The present invention relates generally to an air/fuel ratio control system for an internal combustion engine. More particularly, the invention relates to a system for correcting the air/fuel ratio of the engine depending upon measured atmospheric pressure.
Generally, fuel is metered into a mixture supply in an induction passage of the internal combustion engine so that the metered fuel amount is proportional to an intake air flow rate in order to keep the air/fuel mixture ratio at a satisfactory value corresponding to engine operating conditions. For modern vehicle engines, catalytic converters are provided which operate to define a range of the air/fuel ratios for preventing or limiting the emission of CO, NOx, etc., in the exhaust gas. In other words, the air/fuel ratio of the mixture is controlled in a range where the catalytic converter works effectively.
As is well known, since the air/fuel ratio is controlled by controlling the fuel metered amount in relation to the amount of air supplied to the mixture supply, the fuel amount to be metered is varied depending not only on the intake air flow rate but also on atmospheric air pressure. Particularly in mountainous areas, atmospheric pressure varies depending on vehicle elevation and thus the intake air amount is varied with respect to that of the metered fuel. In order to keep the mixture within the effective range of the catalytic converter, it is, therefore, required to correct the fuel metering amount depending on atmospheric air pressure.
Conventionally, such correction is effected by a mechanical device, such as a pressure responsive diaphragm actuator. Since such mechanical correction involves significant time lag, it permits the mixture to temporarily be too rich for the efficient operation of the catalytic converter.
Further, the mechanical correction device, such as a diaphragm actuator, cannot follow the relatively delicate variation of atmospheric air pressure. Therefore, such conventional correction devices are not accurate for satisfactory engine control. In addition, a conventional mechanical device is apt to vary in its response characteristics while it is used in engine control over a relatively long period of time. This response variation and lacking of durability are disadvantageous and inconvenient and requires periodic maintenance or adjustment of the mechanical device.